Bicycle wheels usually comprise a peripheral crown or rim, a central hub, and a plurality of spokes that connect the hub to the rim. Rims are generally made up of two side walls connected or jointed at one end by a circumferentially inner wall or lower bridge and at an intermediate point by a circumferentially outer wall or upper bridge. This arrangement gives the rim a substantially upside down A-shaped cross-section. The circumferentially outer portions of the side walls, typically have an outer edge that forms a channel for coupling with a tire, whereas the circumferentially inner portions of the side walls form, together with the lower bridge and the upper bridge, a chamber for the attachment of the spokes.
The spokes are attached to seats in the lower bridge or in the inner side walls. The seats generally consist of openings, smooth holes, or threaded holes depending on the method used for the attachment of the spokes.
Rims are manufactured by extruding aluminum alloy bars. The bar is shaped as a circle typically by calendering and its top ends are jointed by welding.
In the field of bicycles, especially in the field of racing bicycles, a common goal is to reduce the weight of a wheel, and in particular a rim, without diminishing its structural strength and static and dynamic stability. Other solutions seek simple and less expensive manufacturing processes that produce aesthetically pleasing and original shapes.
In rim design, it is commonly known that the rim is in the most tension in the (areas of the spoke attachment zones and this tension progressively decreases moving away from such zones. With the goal of reducing the weight of the rim, rim designs may have a greater thickness where the tensions are greater, i.e. in the spoke attachment zones, and a lower thickness where the tensions are lower, i.e. in the zones between the spoke attachment zones (the “infra-spoke zones”).
Several solutions to this problem have been designed: a first known solution to the problem of rim design is described in U.S. Pat. No. 452,649 to Powell; a second solution in patent application PCT WO93/09963; a third solution in European patent EP 715,001; and a fourth solution in European patent EP 1,084,868.
In particular, PCT WO93/09963 obtains the two different thicknesses of the rim (in the spoke attachment zones and in the infra-spoke zones) starting from a rim having a lower bridge with the desired minimum thickness and thickening the spoke attachment zones with a drawing mechanical process. In European patents EP 715,001 and EP 1,084,868, by contrast, the two thicknesses start from a rim with a thickness of the lower bridge substantially equal to that desired in the spoke attachment zones and then material is removed (by chemical processing in EP 715,001 and by mechanical processing in EP 1,084,868) in the infra-spoke zones of the lower bridge.
In all the known solutions, the variations in thickness between the spoke attachment zones and the infra-spoke zones have a transition zone with a substantially curvilinear progression and different radii of curvature which range from a minimum of a few millimeters in EP 715,001 to a maximum of 100 mm in EP 1,084,868. As explained in EP 1,084,868, the progression of such a transition zone is “required” because the tensions introduced by the spokes in the rim are distributed in the rim itself with gradual progressions and “sudden” variations in thickness would lead to concentrations of tensions in the rim and, therefore, to its breaking generally by fatigue.
Moreover, such a requirement is thought of as common knowledge to people skilled in the art, who know that in order to distribute tensions, it is necessary to provide soft and gradual variations in cross-section in order to avoid point stresses.
A drawback of the known solutions is that the curvilinear progression of the variation in thickness does not allow for as great a removal of material, thus leading to a reduction in the weight of a rim not completely satisfactory. Moreover, such a curvilinear progression also defines the shape that must be given to the variation in thickness.